Online Resources

SAGE — Free and open source extension to Python for advanced mathematical work/computation, can be run in-browser.
Pari/GP — Free, and somewhat better for abstract algebra than SAGE, but there's a somewhat steeper learning curve for the syntax. Can be run in-browser.
Magma — Difficult syntax, but probably the most complete (non-graphical) computer algebra system. Stanford University has a license paid for by the Simons Foundation.
Mathematica — A more graphically-driven computer algebra system. It is particularly useful for discrete math/combinatorics, calculus/analysis, and making pretty pictures.
Wolfram Alpha — Useful if you want to check your work or solve a system of equations quickly.

Github — A highly functional collaborative environment for working on documents in a group with solid version control.
Overleaf — For collaboration and cloud storage of LaTeX documents.
Google Drive — Cloud storage. You all have accounts through Stanford, and through Gmail if you use it.
Dropbox — Yet another cloud storage service.
Slack — A chatroom-type environment for collaborative work or just socializing. Has LaTeX compatibility.

Wikipedia — Has reasonably good coverage of mathematics.
Mathworld — An online math encyclopedia specialized towards certain interests by the same company as Mathematica so it tells you how to do things in Mathematica.
MathSciNet — The AMS' literature search engine.
Arxiv — A free online repository of scientific articles. Probably 60-70% of math articles make it to the Arxiv before publication.
Math Stack Exchange — A forum for more elementary questions.
Math Overflow — A forum for more advanced questions.
OEIS — The Online Encyclopedia of Integer Sequences! Possibly the most useful website ever.